The present invention relates to a process for crosslinking ethylene polymers containing anhydride functions, crosslinkable polymer compositions and the application of these compositions to the coating of substrates.
Various processes are known and employed industrially for crosslinking polyethylene under effective conditions, but they have disadvantages which restrict their use. The process of incorporating sources of free radicals (peroxide) is difficult to carry out because:
(1) an excess of reactant (1 to 3% by weight) is employed to compensate for the fairly low yield of bridge formation and losses of reactant caused by the antioxidants or anti-aging additives;
(2) the addition of the peroxidic reactant requires that temperature be rigorously controlled to avoid a premature reaction involving decomposition of the reactant and partial crosslinking of the polyethylene, because the initial reaction has a high activation energy (30,000 to 40,000 calories per mole) and a rise in temperature of 10.degree. C. is sufficient to cause a major acceleration of the reaction;
(3) the reaction is slowed down considerably between 10.degree. and 100.degree. C. because of the high activation energy involved, and it is impossible to obtain the reaction after shaping or forming and cooling. The reaction must therefore be accelerated by working at a high temperature, such as 190.degree. to 220.degree. C., to carry out the reaction in 30 to 60 seconds, which requires large equipment operating under demanding conditions, e.g., a 100 meter long oven for coating electric cables, heating by steam or gas circulation under a pressure from 10 to 20 bars, continuous delivery under pressure, and the like; and
(4) there is a very restricted choice of antioxidants or anti-aging additives that will not inhibit crosslinking so that moderately good aging resistance properties can be obtained.
Another process involves grafting a reactant of the vinyltriethoxysilane type on the polyethylene. In this process, it is possible to crosslink under milder conditions, but the following difficulties arise:
(1) the grafting requires kneading with the silane reactant and a small quantity of peroxide initiator. Kneading makes the reaction sensitive to small amounts of inhibitor and results in a product of uneven quality which may not be sufficiently crosslinkable;
(2) if the quality of the grafted polymer is to be monitored, the latter must be stored after granulation and cooling, which requires very severe storage conditions to prevent the stock from being crosslinked by moisture. If, however, granulation and storage are avoided, there is a risk of producing costly cables which may not be sufficiently crosslinked;
(3) shaping or forming by extrusion must be followed by extended storage in a warm and moist environment to ensure fairly rapid crosslinking (24 h) at low thicknesses (1 to 3 mm); and
(4) aging resistance will be good only if the anti-aging additives are injected after grafting but before shaping or forming.
Olefin polymers or copolymers containing structural units derived from maleic anhydride are already known. U.S. Pat. No. 3,637,579, for example, describes an adhesive composition containing a copolymer of a straight-chain mono-.alpha.-olefin and maleic anhydride, a liquid polyepoxide and a pulverulent filler. The copolymer described must be considered to be a polyanhydride and not a copolymer with a low concentration of maleic anhydride, because the macromolecular structure contains as many .alpha.-olefin structural units as anhydride structural units. The compositions according to this patent cannot be employed in the fabrication of cable coatings.
U.S. Pat. No. 3,760,031 also relates to adhesive compositions, which are intended to promote the adhesion of ethylenevinyl acetate copolymers to a solid material, incorporating (A) a grafted product obtained by reacting an ethylene-vinyl acetate copolymer, or a derivative, with an unsaturated carboxylic acid or its anhydride and (B) a compound containing at least two epoxide groups. The grafted product (A) is thermally unstable and releases corrosive acetic acid between 150.degree. and 250.degree. C., which is undesirable for products which must be stable when exposed to high temperatures.